Internal configuration for a radial heat pipe

ABSTRACT

A radial heat pipe has a plurality of radial struts covered with capillary lining which extend from an outer cylindrical heat output wall to an inner cylindrical heat input wall to assist in returning a working fluid from the heat output wall to the heat input wall. A deformable metallic lining is attached to the outside of the cylindrical heat input wall to provide a close fit with and a good thermal path from a heat source member to the heat input wall. The heat pipe also has a tube extending down its central axis which permits force to be applied to the heat source member to separate the heat source member from the heat pipe.

United States Patent 3,658,125 Freggens [451 Apr. 25, 1972 [541 INTERNAL CONFIGURATION FOR A OTHER PUBLICATIONS RADIAL HEAT PIPE Busse, C. A. Optimization of Heat Pipe Thermionic Conver- [72] Inventor: Robert A. Freggens, Lancaster, Pa. tars, 1965 Eumtom R PP- 1 and Deverall, J. E. et al. H1gh Thermal Conductance Devices, Asslgnw RCA Corporation 4/1965, Los Alamos Scientific Laboratory LA 32] 1, pp. 1 and 22 Filed: Jan. 11,1968

[2]] App]. No.; 697,181 Primary Examiner-Albert W. Davis, Jr.

t Attorney-Glenn H. Bruestle [52] U.S.Cl ..l/80,l65/l05, 165/133, 57 ABSTRACT 313/12 313/44 317/234 A A radial heat pipe has a plurality of radial struts covered with [51] lnt.Cl. ..F28d 15/00, H01 7/28 ca ma min which extend from an 0 er d l h t [58] Field oiSearch ..l65/105, 62/514; 313/12, H t g lb CY 313/44 317/234 A ou put we 0 an inner cy in ma eat mput wa to assist 1n returnmg a working fluid from the heat output wall to the heat input wall. A deformable metallic lining is attached to the out- References cued side of the cylindrical heat input wall to provide a close fit with and a good thermal path from a heat source member to the UNITED STATES PATENTS heat input wall. The heat pipe also has a tube extending down 3,405,299 10/1968 Hall et al ..165/ x its central axis which Permits force to he applied to the heat source member to separate the heat source member from the FOREIGN PATENTS OR APPLICATIONS heat P 965,696 8/1964 Great Britain ..317/234 11 Claims, 2 Drawing Figures 0 a? 91 r 1 1 1 Z/ 9 1 7 7 f I 1 I -Z8 l 1 l 1 w-f I I 7642/ 45% 78 4 I g I A I 2 4:50

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12 35 1 b I r% j MI I} ll L/ t PATEMEDAFMS 1972 3,658,125

sum 2 BF 2 finer/4, 25465:

INTERNAL CONFIGURATION FOR A RADIAL HEAT PIPE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to an improved heat exchanger for electron tubes and particularly to a heat exchanger incorporating the principles of a heat pipe in which heat transfer is effected radially.

2. Description of the prior Art One type of heat pipe is a vapor device which is used to convey heat radially outward from a central heat source member such as an electron tube to surrounding heat dissipation means. This type of heat pipe comprises, for example, a closed annular structure having a lining of capillary material, and a vaporizable working medium therein. The closed annular structure is defined by an inner cylindrical wall and a coaxial outer cylindrical wall radially spaced from the inner wall. Disklike top and bottom walls connect the ends of the inner and outer cylindrical walls and complete the annular structure. The capillary lining covers the inner surfaces of the walls surrounding this annular structure and may consist of wire mesh or porous matrix material. The working medium saturates the capillary lining and is selected to have a vaporization temperature equal to the operating temperature of the heat pipe. For most efficient operation, all undesirable foreign gases are removed from the annular space which constitutes the active area of the heat pipe and the working medium.

In this type of heat pipe, which may be called a radial heat pipe, the exterior surface of the heat input wall forms the wall of a central cylindrical heat input cavity. The heat source member to be cooled is inserted into this central heat input cavity and thereby placed in thermal contact with the exterior surface of the heat input wall of the heat pipe. Some electronic devices such as the collector of a klystron and other external anodes of power tubes have a configuration which makes them especially suitable for cooling by a radial heat pipe, since they can be designed to fit tightly into the heat input cavity.

The heat pipe operates as an efficient heat flux transformer by transferring heat from the smaller surface area of a heat source to a larger area which is suitable for air cooling. Heat from the heat source is conducted through a metallic heat input wall and causes the liquid working medium which saturates the capillary lining on the interior surface of the heat input wall to vaporize and till the annular space between the heat pipe walls. The vaporized working medium then condenses on the capillary lining of the outer or heat output wall of the heat pipe giving up its heat of vaporization. In this way, heat is transferred from the heat source to an area where it can be effectively dissipated to the ambient by radiation, convection or other conventional means.

I-Ieretofore, the heat transfer ability of heat pipes utilizing this radial heat transfer configuration has been limited by an inability to replenish the supply of liquid working medium in the heat input wall quickly enough. Liquid working medium is transported from the heat output wall where it condenses, to the heat input wall by the pumping action of the capillary lining. In order to reach the heat input wall after it has condensed on the heat output wall of the heat pipe, the liquid medium must flow across the disklike top and bottom walls of the annular structure. This is a relatively long distance and since the pumping action of the capillary lining is slow, it can require a relatively long time to return the liquid working medium to the heat input wall where it can be reused. This limits the amount of heat which can be transferred from the heat source member.

Another factor which has limited the effectiveness of prior art radial heat pipes is the inability to insure good thermal contact between the exterior wall of the heat source member and the walls of the heat input cavity. It is always difficult to insure good contact between two cylindrical surfaces but a good thermal contact must exist between these surfaces if heat is to be transferred efficiently from the heat source member to the working medium within the capillary lining on the interior of the heat input wall. Good thermal contact can be provided by brazing the heat source to the exterior surface of the heat input wall or by utilizing the outer wall of the heat source as the heat input wall of the heat pipe, but both of these methods require that the heat pipe be permanently attached to the heat source member. This makes it impossible to reuse the heat pipe and it must be discarded with the heat source member when the latter reaches the end of its useful life. Since the heat pipe has a longer useful life than the heat source member, it would be economically advantageous to be able to remove the heat pipe and reuse it elsewhere.

SUMMARY OF THE INVENTION The foregoing problem of returning the liquid working medium from the heat output to the heat input wall of a radial heat pipe is overcome by the addition of a plurality of radially extending struts between the outer cylindrical heat output wall of the heat pipe and the inner cylindrical heat input wall of the heat pipe. For further increasing the efficiency of the heat pipe, the radial struts may be suitably inclined with respect to the operational position of the heat pipe.

An improved thermal contact may be provided between the heat source member and the heat pipe which will allow the heat pipe to be removed readily from the heat source. Means to apply pressure to the heat source member to further facilitate the removal of the heat pipe may also be included in the heat pipe.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a preferred embodiment of an improved radial heat pipe; and

FIG. 2 is a sectional view taken along 2-2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The heat pipe 10, shown in FIG. 1 is an annular structure which transfers heat outwardly from a cylindrical heat input wall 12 to a surrounding cylindrical heat output wall 14 where the heat is dissipated. The heat output wall 14 is spaced radially from the heat input wall 12 forming an annular space 16 in the area between these walls. A disklike bottom wall 18 connects the heat input wall 12 and the heat output wall 14 closing one end of the annular space 16. As is shown in FIG. I, the heat output wall 14 extends beyond the heat input wall 12 allowing heat from a heat source member to be transferred to a relatively large surface area which is suitable for air cooling. If desired; the heat output wall 14 may be made coextensive with the heat input wall 12, but in this case more efficient cooling means such as liquid cooling should be employed to dissipate heat from the exterior of the heat output wall 12.

A centrally located heat input cavity 20 is adapted to receive a heat source member to be cooled and is designed to provide good thermal contact between the heat source member and the exterior of the heat input wall 12. The sides of the cylindrical heat input cavity 20 are defined by the exterior surface of the heat input wall 12 and are coextensive therewith. The cavity 20 has an opening 22 surrounded by the disklike bottom wall 18 through which a heat source member may be inserted. The cavity 20 is terminated at its upper end, as viewed in FIG. 1, by a disklike member 24. The disklike member 24 having a central opening 25 is attached to one end of the heat input wall 12 and acts as an additional heat input area. A thin lining 26 of deformable thermally conducting material, such as gold or aluminum foil, is attached to the surface of the heat input cavity 20 to insure good thermal contact between the heat source member and the walls of the heat input cavity 20.

The portion of the heat pipe structure above the heat input cavity 20 comprises a second annular area 27 which is defined by the cylindrical heat output wall 14 and a central tubular structure 28. The tubular structure 28 is coaxial with the heat output wall 14 and is connected at one end to the disklike member 24 at the top of the heat input cavity 20. A disklike top wall 30 having a central opening 31 connects the other end of the tubular structure 28 with the surrounding heat output wall 14 and closes one end of the second annular structure 27.

The tubular structure 28 allows pressure to be exerted on the top of a heat source within the heat input cavity 20 by a threaded rod 32 to force the heat source member out of the cavity 20 and thereby disconnect the heat pipe from the latter member. The tubular structure 28 is open at both of its ends. One end portion of the tubular structure 28 extends through the central opening 31 in the top disklike wall 30 and the other end portion extends through the central opening 25 in the disklike member 24 and opens into the heat input cavity 20. Screw threads 34 extend over a portion of the inner surface of the tubular structure 28. The threaded rod 32 may be inserted into the tubular structure 28 through the opening in the top wall 31, and by utilizing the mechanical advantage of the screw threads can exert a large axial force on a heat source member within the input cavity 20.

All the interior walls of the heat pipe structure are lined with a layer of capillary material 36 which stores a liquid working medium and transports the liquid working medium throughout the heat pipe by capillary action. The capillary lining 36 may be made, for example of wire mesh or porous matrix material. The working medium in liquid form saturates the capillary lining 36 of the heat pipe and vaporizes in those areas where heat is applied. A liquid such as mercury, vaporizable at the operating temperature of the heat pipe, can be utilized as a working medium.

A plurality of thermally conducting radial fins 37 are attached to the exterior surface of the heat output wall 14 to dissipate heat from the heat output wall. Forced air can be directed through the fins 37 by any conventional means if additional cooling is desirable.

A plurality of radial struts 38 extend across the annular space 16 and connect the capillary lining on the heat output wall 14 with the capillary lining on the heat input wall 12 to facilitate the movement of liquid working medium from the heat output wall to the heat input wall. In the preferred embodiment shown in FIGS. 1 and 2, the radial struts 38 comprise channels having a metallic base strip 40 and lateral metallic sides 41 perpendicular to the base strip 40. A strip of capillary material 42 is attached to the base strip 40 between the perpendicular walls 41. One end of each of the struts 38 is attached to a narrow capillary ring 44 extending inwardly from the heat output wall 14 and the other end is attached to a narrow capillary ring 46 which extends outwardly from the heat input wall 12. Capillary strips 42 within the struts 38 are contiguous with the capillary rings 44 and 46.

In the preferred embodiment as shown in FIGS. 1 and 2, the heat pipe has 16 radial struts or channels 38 arranged in four levels 48, 50, 52 and 54 having four channels in each level. The channels in each of the four levels are equally spaced circumferentially around the heat input wall 12. The channels in each of the four levels are also equally spaced circumferentially from the adjacent channels which are in the adjacent levels. In this way, each of four channels in each quadrant of the annular space 16 is in a different level and the channels are made sufficiently narrow so that spaces 56 are provided between the channels 38. This arrangement permits the channels 38 to provide a uniform flow of liquid working medium from the heat output wall 14 to the heat input wall 12 and yet avoids hindering the vapor circulation in the heat pipe to a significant extent.

In the preferred embodiment shown in FIG. 1, the radial channels 38 slope downwardly with respect to the operational position of the heat pipe from the heat output wall 14 to the heat input wall 12 so that gravity aids the flow of the working medium in liquid form toward the heat input wall 12 when the heat pipe is oriented as shown in FIG. 1. An angle of from about 10 to about 80 with respect to the vertical position as shown in FIG. l is sufficient to provide the proper flow of liquid working medium to the heat input wall 12. The exact angle of inclination for best results depends on the capillary material used in the channels, the fluid viscosity of the working medium and the length of the channels 38.

This heat pipe configuration has been used to cool the collector of a klystron (not shown). The collector of the klystron may be fitted tightly into the cylindrical heat input cavity 20 so that there is good thermal contact between the collector and the walls of the heat input cavity 20. Heat from the collector is conducted through the metallic wall of the heat input cavity 20 causing the working medium within the capillary lining 36 on the heat input wall 12 to vaporize and expand throughout the interior of the heat pipe. As the working fluid vaporizes, it absorbs heat, thereby cooling the collector. The vaporized working fluid condenses on the heat output wall 14 giving up its latent heat of vaporization. This heat is then dissipated by the metallic fins 37 attached to the exterior of the heat output wall 14.

In order to provide continuous cooling for the collector, the supply of liquid working medium in the capillary lining 36 of the heat input wall 12 must be constantly replenished. The radial channels 38 provide a direct connection across the annular space 16 between the capillary lining 36 on the heat output wall 14 and the capillary lining on the heat input wall 12. These channels 38 greatly decrease the distance which the working fluid must flow through the capillary lining 36 to reach the heat input wall 12 from the heat output wall 14 since it is no longer necessary for the liquid working medium to flow only through the capillary lining on the disklike bottom wall 18 or top wall 31). inclining the channels 38 downwardly from the heat output wall 14 to the heat input wall 12 further increases the return flow of working liquid.

The spacial arrangement aforementioned of the radial channels 38 contributes to a uniform and relatively rapid distribution of liquid working medium to all points on the heat input wall 12. After the vaporized working medium condenses on the heat output wall 14, it flows through the capillary lining 36 until it reaches one of the capillary rings 44. Since the rings 44 extend through the capillary lining on the heat output wall 14, the flow in the capillary wall lining 36 is interrupted and the liquid then flows along the capillary ring 44 until it reaches one of the radial channels 38. Capillary action in the rings 44 will cause the liquid working medium to flow into the contiguous capillary strips 42 on the channels 38 and move through the channel 38 toward the capillary rings 46 on the heat input wall 12. If the channels 38 are inclined downwardly from the heat output wall 14 to the heat input wall 12 gravity will aid capillary action to move the liquid working medium through the channels 38.

Since the capillary rings 46 are located at four different levels on the heat input wall 12, they serve to distribute the liquid uniformly to both the top and bottom of heat input wall. Since the capillary rings 46 extend around a circumference of the heat input wall 12, the liquid working medium will flow along them to all points on the surface of the heat input wall 12 and not just to the areas adjacent to the channels 38. Distributing the liquid working medium uniformly to all points on the heat input wall 12 provides more uniform cooling for the heat source and improves the overall heat transfer capability of the apparatus.

While the radial struts 38 have been shown in a channel structure in the preferred embodiment, other strut configurations will perform satisfactorily. FOr example, a planar strut having no lateral sides 41 can be used rather than the channels shown. If the strip of capillary material 42 is rugged enough to be self-supporting, it can be used without the underlying supporting strip 40 or the lateral sides 41 so that the entire strut is made of capillary material.

In the preferred embodiment of the radial heat pipe, as shown in FIG. 1, the walls defining the cavity 20 are lined with a thin metallic lining 26 to provide good thermal contact between the heat source and the exterior of the heat input wall 12. The thin deformable metallic lining 26 may be a gasket which is attached as by brazing to the walls of the cavity 20. The metal gasket 26 may be a thin layer of a malleable metal such as gold or aluminum. Alternatively, a thin layer of such a malleable metal may be plated onto the surface of the cavity 20. In either case, this thin layer of metal will deform easily, so that it readily accommodates itself to the respective shapes of the heat source member, not shown, and the cavity and thereby provides good thermal contact with the heat input wall 12. To further improve thermal contact between the heat source and the heat input wall 12, the cylindrical sides of the cavity 20 may be tapered inwardly from opening 22 at a small angle of about 1 for example. This slight tapering will provide a truncated cylindrical surface on which good contact will exist between the cylindrical surfaces of the heat source member and the heat input wall 12.

The presence of the central threaded tube 28 facilitates the removal of the heat pipe from a heat source member. Surface diffusion occurring over an extended period of high temperature operation tends to fuse the exterior metal surfaces of the heat source member and of the walls of cavity 20 making it difficult to separate the two A threaded rod 32 may be inserted through the opening 31 in the ringlike top wall 30 and into the tubular structure 28. This rod extends through the tube 28 and exerts pressure against the top of the heat source member through the opening in disklike member 24. The mechanical advantage derived from the screw threads allows sufficient pressure to be brought to bear at the center of the heat source member to cause it to be removed from the heat pipe. Since the heat pipe has a longer potential useful life than the klystron collector, it is advantageous to be able to remove the heat pipe so that it is available for reuse.

What is claimed is:

l. A heat pipe radially arranged about a central vertical axis for cooling a portion of a heat source member, comprising:

a. a first cylindrical wall circularly arranged about said axis b. a second cylindrical wall within and coaxial with said first cylindrical wall,

said first and said second cylindrical walls being spaced radially to define an annular space therebetween;

0. means for closing said annular space within a gas-tight envelope;

d. a capillary structure engaging the inner surfaces of said first and said second cylindrical walls and said closing means;

e. a working medium within said annular space that is vaporizable only at and above the operating temperature of said heat source member;

f. a plurality of radial struts extending between said first and said second cylindrical walls,

said radial struts being arranged in a plurality of discrete levels along said vertical axis, each of said struts being circumferentially spaced from the struts in the adjacent levels so that no strut is located vertically above any part of another strut in an immediately adjacent lower level; and

g. capillary means disposed on at least one surface of said radial struts and connected at each end thereof to said capillary structure.

2. A heat pipe as described in claim 1 wherein said radial struts are arranged in four levels having four radial struts in each level,

said struts in each level being equally circumferentially spaced around said second cylindrical wall.

3. A heat pipe as described in claim 2 wherein said radial struts are equally circumferentially spaced around said second cylindrical wall.

4. A heat pipe as described in claim 1 including a first group of capillary rings extending into said annular space from said first cylindrical wall and a second group of capillary rings extending into said annular space from said second cylindrical wall,

each of said radial struts extending between one of said first group of capillary rings and one of said second group of capillary rings.

A heat pipe as described in claim 1 wherein said radial struts comprise channels, each of which has a bottom and two opposed lateral sides.

6. A heat pipe radially arranged about a central vertical axis for cooling a portion of a heat source member, comprising:

at a first cylindrical wall circularly arranged about said axis;

b. a second cylindrical wall within and coaxial with said first cylindrical wall,

said first and said second cylindrical walls being spaced radially to define an annular space therebetween;

c. means for closing said annular space within a gas-tight envelope;

a capillary structure engaging the inner surfaces of said first and said second cylindrical walls and said closing means;

e. a working medium within said annular space that is vaporizable only at and above the operating temperature of said heat source member;

a plurality of radial struts extending across said annular space between said first and said second cylindrical walls and inclined with respect to said vertical axis; and

g. capillary means disposed on at least one surface of said radial struts and connected at each end thereof to said capillary structure.

7. A heat pipe as described in claim 6 wherein said radial struts slope downwardly from said first cylindrical wall to said second cylindrical wall at an angle of between about 10 and with respect to said central vertical axis.

8. A heat pipe for cooling a part of an electron tube, comprising:

a. a first cylindrical wall;

b. a second cylindrical wall within and coaxial with said first cylindrical wall, said first and second cylindrical walls being spaced radially to define an annular space therebetween;

c. means for closing said annular space within a gas-tight envelope, said closing means including:

1. a disklike member attached to one end of said second cylindrical wall such that said disklike member and said second cylindrical wall define a cavity, said cavity being adapted to receive said part of said electron tube to be cooled, and the walls of said cavity having a relatively thin deformable metallic lining thereon; and

2. a third cylindrical wall having two open ends, one end of which opens into said cavity through an opening in said disklike member, and the other end of which is accessible from the exterior of said heat pipe; and

d. means including a capillary structure within said annular space for transferring heat from said second cylindrical wall to said first cylindrical wall.

9. A heat pipe as described in claim 8 wherein the side walls of said cavity are tapered to insure improved contact with said part of said electron tube.

10. A heat pipe as described in claim 8 wherein said thin deformable metallic lining comprises a gasket of thin malleable metallic foil.

11. A heat pipe as described in claim 8 wherein said thin deformable metallic lining comprises a thin layer of metal plated on the inner surface of said cavity. 

1. A heat pipe radially arranged about a central vertical axis for cooling a portion of a heat source member, comprising: a. a first cylindrical wall circularly arranged about said axis ; b. a second cylindrical wall within and coaxial with said first cylindrical wall, said first and said second cylindrical walls being spaced radially to define an annular space therebetween; c. means for closing said annular space within a gas-tight envelope; d. a capillary structure engaging the inner surfaces of said first and said second cylindrical walls and said closing means; e. a working medium within said annular space that is vaporizable only at and above the operating temperature of said heat source member; f. a plurality of radial struts extending between said first and said second cylindrical walls, said radial struts being arranged in a plurality of discrete levels along said vertical axis, each of said struts being circumferentially spaced from the struts in the adjacent levels so that no strut is located vertically above any part of another strut in an immediately adjacent lower level; and g. capillary means disposed on at least one surface of said radial struts and connected at each end thereof to said capillary structure.
 2. a third cylindrical wall having two open ends, one end of which opens into said cavity through an opening in said disklike member, and the other end of which is accessible from the exterior of said heat pipe; and d. means including a capillary structure within said annular space for transferring heat from said second cylindrical wall to said first cylindrical wall.
 2. A heat pipe as described in claim 1 wherein said radial struts are arranged in four levels having four radial struts in each level, said struts in each level being equally circumferentially spaced around said second cylindrical wall.
 3. A heat pipe as described in claim 2 wherein said radial struts are equally circumferentially spaced around said second cylindrical wall.
 4. A heat pipe as described in claim 1 including a first group of capillary rings extending into said annular space from said first cylindrical wall and a second group of capillary rings extending into said annular space from said second cylindrical wall, each of said radial struts extending between one of said first group of capillary rings and one of said second group of capillary rings.
 5. A heat pipe as described in claim 1 wherein said radial struts comprise channels, each of which has a bottom and two opposed lateral sides.
 6. A heat pipe radially arranged about a central vertical axis for cooling a portion of a heat source member, comprising: a. a first cylindrical wall circularly arranged about said axis; b. a second cylindrical wall within and coaxial with said first cylindrical wall, said first and said second cylindrical walls being spaced radially to define an annular space therebetween; c. means for closing said annular space within a gas-tight envelope; d. a capillary structure engaging the inner surfaces of said first and said second cylindrical walls and said closing means; e. a working medium within said annular space that is vaporizable only at and above the operating temperature of said heat source member; f. a plurality of radial struts extending across said annular space between said first and said second cylindrical walls and inclined with respect to said vertical axis; and g. capillary means disposed on at least one surface of said radial struts and connected at each end thereof to said capillary structure.
 7. A heat pipe as described in claim 6 wherein said radial struts slope downwardly from said first cylindrical wall to said second cylindrical wall at an angle of between about 10* and 80* with respect to said central vertical axis.
 8. A heat pipe for cooling a part of an electron tube, comprising: a. a first cylindrical wall; b. a second cylindrical wall within and coaxial with said first cylindrical wall, said first and second cylindrical walls being spaced radially to define an annular space therebetween; c. means for closing said annular space within a gas-tight envelope, said closing means including:
 9. A heat pipe as described in claim 8 wherein the side walls of said cavity are tapered to insure improved contact with said part of said electron tube.
 10. A heat pipe as described in claim 8 wherein said thin deformable metallic lining comprises a gasket of thin malleable metallic foil.
 11. A heat pipe as described in claim 8 wherein said thin deformable metallic lining comprises a thin layer of metal plated on the inner surface of said cavity. 